A driver of a Tesla car has mentioned in this thread that the car sometimes doesn't register the hands on the wheel.
The car should be conservative about the threshold it uses for the sensors in determining if there are hands (because it would rather err on the side of telling someone to put their hands back on, than err on the side of thinking someone has their hands on when they don't), but engineers analyzing the logs after a crash wouldn't have to use that same threshold when trying to determine if the driver was in control of the car at the time of the crash.
Plus, it's a pretty common complaint in Tesla forums.
On the other hand, this is effectively an admission by Tesla that there are serious problems with the autopilot system. It should never guide the car into a barrier, no matter where the driver's hands are.